Beneficiation of cryolite material



United States Patent 3,493,330 BENEFICIATION 0F CRYGLITE MATERIAL Donald Otis Vancil and Maurice Clark Harrison, Longview, Wash., assignors to Reynolds Metals Company, Richmond, Va., a corporation of Delaware No Drawing. Filed Nov. 22, 1966, Ser. No. 596,087 Int. Cl. C01f 7/54; C22d 3/12 US. Cl. 23-88 11 Claims ABSTRACT OF THE DISCLOSURE Cryolite having a weight ratio of NaF to AlF less than 1.5, useful as an additive for alumina reduction cells, is prepared by reacting cryolite material in the presence of reactive alumina with an acid reactant having a fluorine-containing component, such as hydrofluoric acid or its alkali salts.

This invention relates to the beneficiation of cryolite materials. More particularl the invention concerns a method of lowering the NaF/A11 weight ratio of cryolite material to less than 1.5 to l, and to the utilization of such lower ratio cryolite in the operation of alumina reduction cells.

In natural cryolite (NaaAl fi), the weight ratio of NaF to A11 is close to 1.5 to 1. In synthetic cryolites, including those produced by off-gas recovery processes which involve reacting sodium fluoride solution and sodium aluminate solution, followed by precipitation of the cryolite by the introduction of carbon dioxide gas, the products are usually characterized by a weight ratio of NaF to AlF much in excess of 1.5, and which may run a high as 1.9, as well as by substantial silica contamination. The hypothetical reaction for the foregoing synthesis is:

Such synthetic cryolite may also contain substantial amounts of alumina and sodium carbonate, sometimes in the form of Dawsonite Al O .Na CO and is thus alkaline in character.

During the operation of alumina reduction cells, the molten cryolite electrolyte gradually becomes depleted in fluoride content, by vaporization of components rich in aluminum fluoride. At the same time cryolite components rich in sodium are absorbed into the carbonaceous cathode of the reduction cell. Since it is generally considered desirable to maintain the NaF to All weight ratio of the electrolyte within the range of about 1.3 to 1.5 for optimum cell operation, soda ash may be added during this period to replace the excess sodium collected in the cathode, and alkaline cryolite is advantageously used for this purpose. This phase of operation will usually occur within six to twelve months after a new cell is placed in use.

For the remaining two to three years of cell life it becomes necessary periodically to add aluminum fluoride and cryolite to match the composition of the vaporization losses. Where the make-up cryolite contains excess sodium fluoride, or sodium oxide, hydroxide or carbonate values, even greater amounts of expensive aluminum fluoride must be added to preserve the ratio. Thus, during the greater part of the operating life of an alumina reduction cell, both cryolite and aluminum fluoride must be supplied to the cell, necessitating the maintenance of in ventories of such compounds, and increasing the cost of operation.

It is also customary to recover fluorine values from cell waste gases, and from used pot linings and other materials "ice employed in the cells. This is usuall accomplished by employing a caustic aluminate leach liquor, followed by carbonation to precipitate synthetic cryolite. The result ing cryolite is also alkaline and of limited usefulness.

It has been proposed in the prior art to beneficiate synthetic cryolite so as to raise its fluoride content and lower its silica content by reacting the precipitated cryolite with a soluble fluoride or with hydrofluoric acid, at elevated temperature, for a period of /2 to 1 /2 hours, and a method of this type is described in Gernes, US. Patent 3,061,411. Under these conditions, however, the weight ratio of NaF to AlF is only slightly lowered, and in any event, remains in the range of 1.65 to 1.73, or well above the value of 1.5, so that the cryolite retains its essentially alkaline character.

Accordingly, the problem of providing a cryolite having a low NaF/AlF weight ratio, and which would be of great value as an additive to alumina reduction cells in maintaining the optimum ratio of NaF to AlF in the electrolyte, as well as for other purposes, remained unsolved prior to the present invention.

In accordance with the present invention, there is pro- Vided a novel method whereby a cryolite material having excess sodium may be beneflciated and converted to a more desirable low ratio cryolite material comprising excess aluminum fluoride, by reacting said cryolite material in the presence of reactve alumina with an acid reactant having a fluorine-containing component such as hydrofluoric acid or an alkali salt thereof.

In the aluminum industry, and for purposes of this invention, the term cryolite material is employed to denote a range of materials comprising mixtures of NaF and A11 or one or more of the double salts of NaF and AlF which double salts may also contain uncombined NaF or AlF The cryolite materials are usually char acterized by the ratio of the total weight of NaF contained in a given quantity to the total weight of AlF present, without regard to the presence or absence of chemical bonding between the NaF and AlF The value of this Weight ratio for naturally occurring cryolite is about 1.5, in good agreement with the weight ratio of the molecular double salt 3NaF.AlF X-ray and other evidence indicates the existence of two other double salts, 5NaF.3AlF (weight ratio 0.833), and NaF.Alf (Weight ratio 0.5).

It is also common in the aluminum industry to characterize cryolite materials by their percent excess AlF or percent excess NaF, which terms are defined as the percent AlF (or NaF) present in the material in excess f the amount required to form, with the NaF (or AlF present, a cryolite of weight ratio 1.5. Thus, the double salts 5NaF.3AlF and NaRAlF would be described as having about 24.2 and 44.4 percent excess Allrespectively.

The cryolite material amenable to the treatment of the invention may be derived from any suitable source, including synthetic cryolite prepared as previously described, as by direct precipitation or by recovery from reduction cell waste gases, or from cell linings and electrodes. Natural (Greenland) cryolite can also have its weight ratio of NaF/A11 lowered by the method of the invention.

The reactive alumina should be in a form which is readily attacked by the acid reactant, as for example, Bayer process alumina trihydrate, sodium aluminate, aluminum hydroxide, or the mineral Dawsonite (AlgOg-NflgCOg) which is commonly present in alkaline cryolite materials which have been precipitated by the reaction of carbon dioxide on caustic-fluoride solutions containing an excess of alumina. The amount of reactive alumina employed must be suflicient to adjust the NazAl molar ratio of the initial cryolite material to less than 3:1, preferably a slight excess of about 2 to alumina in order to achieve optimum fluorine recovery.

Thus, for example, if it is desired to reduce the ratio of relatively pure cryolite, the appropriate amount of Dawsonite, or preferably freshly precipitated alumina trihydrate, may be added to the water slurry of the cryolite.

If desired, alumina in excess of the amount required for reaction 1) above may be added at any point in the cryolite production system where such alumina might reasonably be present, or be produced either ahead of, or in, the carbonation equipment used for reaction (1).

There may be employed as the acid reactant, in accordance with the invention, either (a) hydrofluoric acid, or (b) HF or an alkali salt thereof, such as sodium fluoride, in admixture with a mineral acid such as hydrochloric, sulfuric, nitric, sulfurous or hypochlorous acids, or an aluminum salt of any such acid. The fluorine-containing component of the acid reactant is used, however, in an amount sufiicient to provide any additional fluorine values needed for the desired final product. Any remaining portion of the total acid reactant required may be chosen from the other reactants mentioned.

It is to be noted that the use of an aluminum salt such as aluminum chloride, for example, avoids the necessity of adding reactive alumina separately, because suflicient reactive alumina can be produced in situ by addition of sodium hydroxide or the like. Thus, the terminology in the presence of reactive alumina is used herein in the general sense of there being provided in one way or another sufiicient aluminum ions to effectuate the desired reaction to form cryolite material comprising excess alumi num fluoride.

A unique feature of the method of the invention lies in the fact that the beneficiated cryolite material is recovered as a solid, while the impurities, and particularly silica impurities, are either taken into solution or remain in solution. The beneficiated cryolite material may then be readily filtered, leaving the sodium and silica in solution.

It is believed that the reactive alumina (together with excess sodium present) is converted to a useful lower ratio cryolite material, e.g. chiolite Na Al F in accordance with the hypothetical reaction:

Or, when treating cryolite material having a 2.5 weight ratio of NaF to AlF for example, to produce beneficiated cryolite of 1.0 ratio, the reaction may be as follows:

It can be seen therefore, that, depending upon the amount of Na CO or other excess sodium values present in the initial cryolite material, the weight proportions of reactive alumina and acid reactant can be estimated in advance. On the other hand, if suflicient reactive alumina is provided to adjust the Na:Al molar ratio of the cryolite material to the desired value less than 3:1, as previously discussed, the reaction can be carried out effectively simply by introducing the acid reactant until stabilization of the pH indicates completion of the reaction. The proportions of acid reactant and reactive alumina can be varied as required to approach the optimum final pH of about 6.0 to 6.5. If too much acid is introduced, so that the pH falls lower than desired, this can be compensated by in troducing an additional amount of cryolite material containing Dawsonite, or other source of reactive alumina and a sodium compound.

By adjustment of the amount of HF added, the proportion of sodium consumed may be regulated to produce an NaF/AlF weight ratio less than 1.5, with a simultane ous decrease in the level of such detrimental impurities as Na CO SiO Fe O and Na SO The pH may drop as low as 3 or 4, depending upon the rate of addition of the HF, but then rises slowly during digestion as sodium carbonate is neutralized. The reaction time will be upwards of about 20 minutes, depending somewhat upon the proportion of acid added and the nature of the material treated. When the pH of the slurry after admixing the HF and allowing time for reaction has reached about 6.0 to 6.5, it is found that about 9-0 to 98 percent of the fluorine originally present is contained in the insoluble lowratio cryolite material produced. Any fluorine dissolved in the liquid portion of the slurry may be recovered by recirculating the liquor through the process.

It is necessary to have the final pH of the slurry within the range of about 4.8 to 6.7 in order to maintain silica in solution while the cryolite precipitates. By thus controlling the pH, it is possible to utilize the fluorine from the acid reactant, as well as excess sodium values in the initial cryolite material, while leaving the silica in solution. The final pH appears to have a definite relation to the NaF/AlF weight ratio, as well as being critical to solubilizing the silica.

The acid reactant treatment temperatures ordinarily will be in the range of about 70 to 100 C., preferably about to C.

After the digestion period is completed, the cryolite product is filtered, and may be dried at not more than about 450 C., depending upon the use to which the croylite is to be put.

The following examples illustrate the practice of the invention, but are not to be regarded as limiting:

EXAMPLE 1 Treatment of high-ratio cryolite with HF 400 g. of cryolite made by reaction of sodium fluoride and sodium aluminate followed by carbonation, having an NaF/AlF weight ratio of 256 (containing 19.2% excess A1 0 and 18.8% excess N21 CO were digested in 1200 ml. of H 0 and 240 ml. of 50% HF for 20 minutes at 85 C., the pH rising from 5.9 to 6.6. Comparative analyses were:

TABLE 1 Percent. Percent Percent Percent NaFjAlF; Sample F SiOg F6203 N82SO4 wt. ratio Untreated 26. 06 U. 46 .05 l. 35 2. 56 Treated v 56. 45 0. 07 O3 0. 25 0. 81

EXAMPLE 2 Treatment of residue black mud from pot lining caustic extraction with HF Reduction cell potliner material (viz. insoluble residue following caustic soda extraction of pot lining to recover cryolite) containing 35% A1 0 1.75% caustic soluble F, 8% CaF 1.5% F6 0,, and 5% SiO was pelletized with Na CO and calcined at 1100 C., to obtain, after leaching with Water, a product practically free from Fe O but heavily contaminated with Na CO and SiO The amount of Na CO was regulated so as to be sufiicient for the production of low ratio cryolite. The reaction may be represented by the equation:

2Na2CO3+ CaFz +A12'O3 +2NaF Casio 200 (3 ficiating cryolite material, by treatment respectively with non-fluorine containing mineral acids and aluminum salts thereof, or with fluosiliceous acid reactants; and the contents of such applications are incorporated herein by reference. One important distinction between these applications is that the method of Ser. No. 596,089 apparently results only in neutralization of any excess sodium compounds present in the initial cryolite material, whereas the method of the present invention enables the recovery of sodium from such compounds in the resulting beneficiated cryolite material. Ser. No. 596,057 involves the use of fluosilicic acid and the alkali salts thereof.

Reference likewise is made to Vancil and Harrison applications Ser. No. 596,088, and Ser. No. 596,236, both filed Nov. 22, 1966 and dealing, respectively, with a method of producing cryolite material by direct preciptiation, and a method of producing beneficiated cryolite material to compensate for reduction cell losses.

While the presently preferred practices of the invention have been described, it will be apparent that the invention may be otherwise variously embodied and practiced within the scope of the following claims.

What is claimed is:

1. Method of lowering the NaF/AlF weight ratio of cryolite material to a value less than 1.5 to l, which comprises reacting said cryolite material in the presence of reactive alumina with an acidic reactant having a fluorinecontaining component selected from the group consisting of hydrofluoric acid and alkali salts thereof, the amount of reactive alumina being sufficient to adjust the NazAl molar ratio of said cryolite material to less than 3:1, said acidic reactant being introduced in an amount such that the reaction mixture reaches a final pH in the range from about 4.8 to 6.7 in order to maintain silica impurities in solution, and recovering the resulting cryolite material in precipitated form.

2. The method of claim 1 in which the initial cryolite material comprises an alkaline synthetic cryolite.

3. The method of claim 1 in which the initial cryolite material comprises a synthetic cryolite containing reactive alumina and sodium carbonate.

4. The method of claim 1 in which said acid reactant is hydrofluoric acid.

5. The method of claim 1 in which the amount of HF or alkali fluoride employed is suflicient to provide any additional fluorine needed to produce cryolite material of the desired NaF/AlF ratio.

6. The method of claim 5 in which the amount of reactive alumina employed is about 2 to 10% by weight (dry basis) in excess of that required to combine with fluorine to form cryolite material of the specified ratio.

7. The method of claim 1 in which the reaction proceeds to a final pH of about 6.0 to 6.5.

8. The method of claim 1 in which said acid reactant contains an amount of HF suflicient to convert at least a portion of the reactive alumina into cryolite comprising excess aluminum fluoride.

9. The method of claim 1 in which the reactive alumina is introduced together with the acid reactant.

10. The method of claim 1 in which the reactive alumina is introduced during the preparation of the initial cryolite material.

11. The method of claim 1 in which the reaction temperature is between about and C.

References Cited UNITED STATES PATENTS 2,186,433 1/1940 Schwemmer 2388 3,049,405 8/ 1962 Trupiano et a1. 2388 3,128,151 4/1964 Zanon et al 2388 3,207,575 9/1965 Garing et al 2388 EDWARD STERN, Primary Examiner US. Cl. X.R. 2352, 110, 

